Thermostatic switches have long been used to provide a protective, temperature responsive switch that automatically breaks contact between a power supply and the wiring of an electrical component upon an increased temperature of the ambient that would subject the component to damage and that automatically re-establishes contact after the ambient has cooled to a safe level of temperature. Thermostatic switches can include a temperature responsive bimetal blade connected, at one end, to an electrically conductive member, such as a terminal arm, with the other end freely extending towards a fixed contact. Upon a change in temperature from the ambient, the central portion of the bimetal blade deflects to move a movable contact, connected to its other freely extending end, to a position either against or spaced from the fixed contact. As can be appreciated, when the contacts are spaced from one another a circuit open condition of the switch is established and when the contacts are against one another a circuit closed condition of the switch is established.
The bimetal blade is designed such that its central portion deflects at a selected operating temperature to produce the circuit open and the circuit closed conditions of the switch. As is well known in the art, the central portion can have a dish-like, snap action depression to produce a snap action of the switch. In such a case, the design features of the blade, including its metallic constituents and the size and the depth of the depression, are carefully selected to form a blade having a predetermined mechanically set, calibration temperature to produce a specific snap point of the blade. Additionally, as with any other dynamic alternately stressed, mechanical component, a bimetal blade is designed for a total number of cyles of deflections between the circuit open and circuit closed positions before failure.
The connection of the blade to the terminal arm, being adjacent to the central flexing portion of the blade, is a critical component of the design of a thermostatic switch because the type of connection chosen can change the carefully engineered operating temperature and projected life of the blade. In the prior art, the preferred method of connecting the blade is to weld the end of the bimetal blade to a terminal arm. Although the weld assures an electrical connection and provides a mechanical connection, the heat generated during the application of the weld weakens the central flexing portion of the bimetal blade. This weakening reduces the projected life of the blade and further shifts the predetermined mechanically set, calibration temperature of the blade. Also in the prior art, bimetal blades have been connected by riveting the blade to a terminal arm. While such a connection does not decrease the projected life of the blade, nor shift its operating temperture, an electrical connection is not assured. These prior art methods of connection are mutually exclusive in that only one or the other method of connection has been used.
In a preferred embodiment of the present invention, a fastener is used to connect the bimetal blade to a terminal arm to prevent the possible weakening of the central flexing portion of the blade as would occur with welding. A weld is also used to connect the bimetal blade to the terminal arm to assure an electrical connection. The weld is, however, situated on the end of the blade so that the fastener is located between the weld and the central flexing portion. The improved connection of the present invention, that utilizes both types of connection, to wit: a fastener and a weld, produces advantages that are not found in either of the methods of connection standing alone. The most important advantage relates to the location of the weld relative to the fastener and the central flexing portion of the blade. In the present invention the weld is remotely located from the central flexing portion of the blade because the fastener is located between the weld and the central flexing portion of the blade. As a result, not only is an electrical connection assured by the weld, but in addition, the central flexing portion is not subjected to elevated temperatures produced during the application of the weld that can detrimentally affect the engineered design characteristics of the blade. As stated previously, the design characteristics of conventionally welded blades are upset by the welding process. Moreover, the fastener being located close to the central flexing portion of the blade assures a stable mechanical connection of the blade. The addition of the weld assures an electrical connection that would not be produced by the fastener connection alone. Thus, a very stable mechanical and electrical connection is provided by the present invention that does not affect the design characteristics of the blade and is not produced in the prior art welded and rivetted connections.